1. Field of the Invention
This invention relates to a liquid developer for developing an electrostatic latent image in the fields of electrophotography, electrostatic printing and the like.
2. Description of the Prior Art
Heretofore, a liquid developer for developing an electrostatic latent image in electrophotography and the like has been that generally prepared in such a manner that a pigment, a dye and the like are kneaded along with a resin, a dispersing agent, a chargeability controlling agent, a solvent and the like to form a paste-like kneaded material and the kneaded material is dispersed in a highly insulating carrier liquid. The dispersing agent and chargeability controlling agent are important components which are used for the purpose of improving chargeability, dispersion stability, preservation stability and the like of the toner in the liquid developer. As such dispersing agent and chargeability controlling agent, linseed oil, soybean oil, rosin, asphalt and the like have been used so far. However, these natural products are not constant in their compositions, and therefore it has been very difficult to prepare a liquid developer which has always a constant property. Further, the liquid developer has had such a drawback that the dispersibility and the charge polarity of the toner particles become poor and unclear during use and preservation thereof for a long period of time.
As a method for controlling the polarity of the toner in the liquid developer, in addition to a method of coating the surface of the pigment particles with various natural products as mentioned above, a method in which a surface active agent is dissolved in the carrier liquid of the liquid developer to cause it to adsorb on the toner to control the charged state of the toner has been known. As the surface active agent used for the purpose, there may be mentioned numerous materials, for example, a metal soap such as cobalt naphthenate, nickel naphthenate, manganese naphthenate and the like, a metal alkylbenzenesulfonate, and a phosphatide such as lecithin, cephalin and the like, but their manners of use and effects are not always constant. Further, these surface active agent are generally of a lower electric resistance so that they inevitably lower the electric resistance of the carrier liquid when they are dissolved in such carrier liquid. On account of this, the surface active agent should be dissolved in the carrier liquid to such an extent that it does not destroy an electrostatic latent image formed on a photosensitive member so that the amount of the surface active agent to be added is subjected strict restriction. Thus, such method using the surface active agent is considered to be defective in that the surface active agent can not be added to the carrier liquid in an amount sufficient to control the chargeability of the toner in the liquid developer.
The materials for controlling the polarity of the toner so as to be negative by dissolving them in the carrier liquid are few, and only lecithin, an alkylbenzenesulfonate, a polyamide resin and the like are known. When these materials are used for a liquid developer as a chargeability controlling agent, they can not impart a sufficiently high interfacial potential to the toner and they change in their qualities owing to the oxidation, for example, due to contact with air while allowed to stand, the oxidizing function of the other components in the toner composition and the oxidizing function of the ozone and the like generating upon corona charging in operation of a copying machine. As the result, the chargeability controlling property of such materials is deteriorated, and therefore the image density and sharpness of the resulting image are liable to lower. In view of the foregoing, the liquid developer containing such materials has had the drawback that it can not be used for a long period of time.